Project Summary Cancer is a collection of complex, heterogeneous, and devastating diseases; however, one common theme is recurrent and high-frequency mutations in the TP53 tumor suppressor gene. In contrast to other tumor suppressors, most TP53 alterations are missense mutations and these exhibit gain-of-function phenotypes that are dependent of transcriptional rewiring. This proposal is focused on the role of p53 missense mutations in triple negative breast cancer (TNBC). Breast cancers are widespread malignancies representing 15% of all cancer deaths in women worldwide. 80-88% of basal-like breast cancers, the majority of which are TNBC, have mutations in the TP53 tumor suppressor. We have recently developed the first genetically engineered mouse model of somatic breast cancer driven by p53 missense mutations. This conditional model represents a major advance as it expresses mutant p53 from the endogenous locus and maintains wild-type p53 in the stroma and immune system. Moreover, our model faithfully mimics the histology of human TNBC, and develops TNBC with a one year latency. Moreover, in these mice, deletion of mutant p53 with CRISPR-Cas9 results in tumor regression and increased survival, indicating that the breast tumors are addicted to mutant p53. In addition, we see increased infiltration of T lymphocytes upon tumor regression. In this study, we propose to understand the biology of TNBC initiated by a somatic p53 driver mutation and determine why these tumors are addicted to mutant p53. This work combines our elegant and disease relevant mouse models of TNBC with state-of-the- art genomic profiling to comprehensively understand the mechanisms underlying mutant p53 driven breast cancer. Moreover, we will use cell lines recently derived from our model that are also addicted to mutant p53 as a platform to undertake a functional genomics screen to identify novel vulnerabilities in these cancers. Immune based therapies have revolutionized cancer care. Therefore to effectively advance mutant p53 directed therapies, a comprehensive understanding of the immune landscape and response is needed. Since our somatic breast model uniquely retains a WT p53 microenvironment and immune compartment, we are in an ideal position to examine the immune response to mutant p53 directed therapies. Our specific aims are: Aim 1. To understand the underlying biology that drives breast cancer development in p53-null versus mutant mice, we will perform whole exome sequence analyses. To identify the molecular pathways that lead to addiction of mutant p53 breast cancers, we will perform RNA-seq analyses of somatic p53R172H breast tumors with and without deletion of mutant p53. Aim 2. To determine the molecular pathways downstream of mutant p53 that are essential for tumor maintenance using CRISPR/Cas9 functional screens. Aim 3. To understand and exploit the immune response to therapeutic inhibition of mutant p53 in breast cancer. Because our conditional mouse model shares the underlying molecular pathology with human sporadic tumors, it will be more predictive of human responses to drugs, and thus a valuable tool in preclinical testing.